An empty frame remains where The Storm on the Sea of Galilee was once displayed. The piece was one of 13 stolen during the 1990 Isabella Stewart Gardner Museum art heist. (Photo: Courtesy of the FBI via Wikimedia Commons)

The Board of Trustees at the Isabella Stewart Gardner Museum in Boston remain optimistic about the 13 missing masterpieces that have eluded federal investigators for 27 years. Just last month, the Board announced they would be doubling their reward, from $5 million to $10 million, for the stolen artworks by renowned painters like Edgar Degas, Edouard Manet and Rembrandt.

But a new report in the Boston Globe shows their optimism may be misplaced. According the Globe, three sources close to the investigation revealed that essential evidence to the case had disappeared.

In the early morning of March 18, 1990, two men walked into Boston's Isabella Stewart Gardner Museum disguised as city police officers. They used duct tape and handcuffs to restrain two museum security guards; the duct tape and handcuffs, which could hold the DNA of the thieves, is reportedly no longer available to the FBI.

The Globe reports that it is unclear when the two key pieces of evidence went missing: sources reportedly told the newspaper that the items had gone missing for more than a decade, and it is unknown if the duct tape and handcuffs were apprehended, misfiled or thrown away.